Many systems and devices rely on clock signals to provide timing control. These clock signals can be provided using crystal oscillators, wherein each crystal oscillator is capable of providing one unique frequency of clock signal. Unfortunately, a crystal oscillator has a high Q value, thereby resulting in a spectral energy of the clock signal being focused in a narrow frequency band. This concentration of energy and its associated harmonics can result in emission of electromagnetic interference in excess of that recommended for those systems and devices.
Modulating the frequency of the clock signal spreads its spectral energy over a wider band of frequencies, thereby reducing the energy of the clock as well as its undesirable harmonics. This modulation is known in the industry as frequency hopping, wherein the frequency varies about a target frequency. (Note that this variation is preferably limited to a relatively narrow band to ensure the overall system timing is preserved.) Reducing the energy (typically measured in decibels with respect to a milliwatt, i.e. dBm) at any one frequency advantageously reduces the noise at that frequency.
Therefore, a need arises for a system and method of modulating the frequency (also referenced herein as providing a spread spectrum clock signal). A need also arises for providing this spread spectrum clock signal in a manner that preserves a uniform delay across temperature.